fix asprintf warnings

fix asprintf warnings

Fix recently introduced compiler warnings:

desktop-shell/shell.c: In function 'shell_configuration':
desktop-shell/shell.c:588:10: warning: ignoring return value of
'asprintf', declared with attribute warn_unused_result [-Wunused-result]

src/screenshooter.c: In function ‘screenshooter_binding’:
src/screenshooter.c:291:10: warning: ignoring return value of
‘asprintf’, declared with attribute warn_unused_result [-Wunused-result]

src/text-backend.c: In function ‘text_backend_configuration’:
src/text-backend.c:944:10: warning: ignoring return value of ‘asprintf’,
declared with attribute warn_unused_result [-Wunused-result]

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>